In these years, various pieces of information are held in information devices as the information devices are rapidly gaining in popularity. Therefore, it is significantly important to establish a security technique for suppressing leakage of information held in the information devices and the like, and various authentication techniques (for example, password authentication, biometric authentication, card authentication, and the like) have been proposed. In most of the known authentication techniques, however, an authentication process is performed only during login, and there is a problem that these authentication techniques have in common in that it is difficult to detect unauthorized use if, for example, an unauthorized third party uses the information device while an authorized user is leaving the position at which the information device is installed.
In order to solve the above problem, continuous authentication techniques have been proposed in which a user is continuously subjected to an authentication process after login. A typical example of the continuous authentication is continuous authentication realized by face recognition using feature vectors. In the face recognition, feature points that serve as the feature vectors of a face region corresponding to a user in an image are registered in advance, and continuous authentication is realized by detecting the user in the image on the basis of the correspondence between the registered feature points and feature points extracted during the authentication. For example, techniques relating to the continuous authentication are disclosed in F. Monrose and A. D. Rubin “Keystroke dynamics as a biometric for authentication”, Future Generation Comput. Syst., vol. 16, pp. 351-359, 2000, A. Altinok and M. Turk “Temporal integration for Continuous Multimodal Biometrics”, in Proc. Workshop on Multimodal User Authentication, 2003 and T. Sim, S. Zhang, R. Janakiraman, and S. Kumar “Continuous Verification using Multimodal Biometrics”, IEEE Trans. Pattern Anal. Mach. Intell., vol. 29, no. 4, pp. 687-700, April 2007.
In addition, color histogram authentication using color histograms of captured images of a user has been proposed. The color histogram authentication is advantageous in that, compared to other methods using face recognition or the like, robust continuous authentication is possible relative to changes in the posture of the user. In the color histogram authentication, for example, a color histogram of a region (for example, a body region) corresponding to a user in an image is registered in advance, and continuous authentication is realized by detecting the body region of the user in the image on the basis of the similarity between the registered color histogram and the color histogram of a captured image and tracking the movement of the body region of the user. For example, a technique relating to the color histogram authentication is disclosed in K. Niinuma, U. Park, and A. K. Jain “Soft Biometric Traits for Continuous User Authentication”, IEEE Transactions on Information Forensics and Security (TIFS), Vol. 5, No. 4, pp. 771-780, 2, 2010.